The reason why that happens (to the best of my knowledge), is because he's maxed out his level, he'll ride around in a limo as sort of a sign of his prestige and status. It's because he's a level 10.
If it really bothers you, a way to get around it is to drop him to a level 9 criminal and then just sort of keep him there:
- open cheat console (shift, ctrl, c)
- type in: testingcheatsenabled true (do these first three steps once you're done and use " false " instead of " true " to turn it off)
- hit enter
- make him your active sim
- shift-click on the mailbox and go to career (can't remember exact wording) and choose criminal and level 9 - whichever one that is
There is some mention here of a certain fix (we're not allowed to discuss that type of fix here), if you're interested, take a look at it: http://forum.thesims3.com/jforum/posts/list/524362.page
